Gas discharge lamp comprising a phosphor layer

ABSTRACT

The invention relates to a gas discharge lamp for dielectrically impeded discharges, which gas discharge lamp is provided with a discharge vessel filled with a gas filling, which discharge vessel comprises at least a wall of a dielectric material and at least a wall having a surface which is at least partly transparent to visible radiation and coated with a phosphor layer, which phosphor layer comprises a phosphor having a host lattice, Eu 2+  as the activator and a doping D selected from the group formed by Ce 3+ , Pr 3+  and Tb 3+ , and provided with an electrode structure for a dielectrically impeded discharge and means for igniting and maintaining the discharge. The invention also relates to a phosphor comprising a host lattice, Eu 2+  as the activator and a doping D selected from the group formed by Ce 3+ , Pr 3+  and Tb 3+ .

[0001] The invention relates to a gas discharge lamp for dielectricallyimpeded discharges, which gas discharge lamp is provided with adischarge vessel filled with a gas filling, which discharge vesselcomprises at least a wall of a dielectric material and at least a wallhaving a surface which is at least partly transparent to visibleradiation and coated with a phosphor layer, and provided with anelectrode structure for a dielectrically impeded discharge, and withmeans for igniting and maintaining the dielectrically impeded discharge.

[0002] Gas discharge lamps for dielectrically impeded discharges areused, particularly, in office automation devices, for example colorcopiers and color scanners, in signal devices, for example as brakelights and direction indicator lights in automobiles, in auxiliaryillumination devices, for example, the courtesy lighting of automobiles,as well as, in particular, for the background lighting of displays anddisplay screens, for example of liquid crystal displays.

[0003] Said applications require the luminance to be both uniformthroughout the length of the lamp and high. To increase the luminance,it is necessary to increase the power that is coupled into the system.However, this also causes a higher load to be imposed on, inter alia,the phosphors in the phosphor layer. The phosphors degrade more rapidlyand the luminous output decreases more rapidly during operation as thepower coupled in increases.

[0004] In particular gas discharge lamps for dielectrically impededdischarges comprising phosphors containing Eu²⁺ as the activator areadversely affected by said degradation of the phosphors. Phosphorscomprising Eu²⁺ as the activator are disclosed in “Ullmans Enzyklopädieder technischen Chemie”, 16, 195 (1978), Weinheim.

[0005] Therefore, it is an object of the invention to provide a gasdischarge lamp for dielectrically impeded discharges, which gasdischarge lamp is provided with a discharge vessel filled with a gasfilling, which discharge vessel comprises at least a wall of adielectric material and at least a wall having a surface which is atleast partly transparent to visible radiation and coated with a phosphorlayer, which phosphor layer comprises a phosphor containing Eu²⁺ as theactivator, and which gas discharge lamp is also provided with anelectrode structure for a dielectrically impeded discharge, and withmeans for igniting and maintaining the dielectrically impeded discharge,in which discharge lamp degradation of the phosphor is reduced.

[0006] In accordance with the invention, this object is achieved by agas discharge lamp for dielectrically impeded discharges, which gasdischarge lamp is provided with a discharge vessel filled with a gasfilling, which discharge vessel comprises at least a wall of adielectric material and at least a wall having a surface which is atleast partly transparent to visible radiation and coated with a phosphorlayer, which phosphor layer comprises a phosphor having a host lattice,Eu²⁺ as the activator and a doping D selected from the group formed byCe³⁺, Pr³⁺ and Tb³⁺, and provided with an electrode structure for adielectrically impeded discharge and means for igniting and maintainingthe discharge.

[0007] In a gas discharge lamp with a phosphor layer, which comprises aphosphor with Eu²⁺ as the activator and a doping D selected from thegroup formed by Ce³⁺, Pr³⁺ and Tb³⁺, the degradation of the phosphor isreduced and the color point of the lamp remains unchanged.

[0008] The invention is based on the recognition that the degradation ofphosphors comprising Eu²⁺ as the activator in gas discharge lamps fordielectrically impeded discharges due to photo-oxidation of said Eu²⁺ iscaused by the VUV radiation from the gas discharge lamp having awavelength in the range from 100 to 200 nm, whereas excitation by meansof light having a longer wavelength in the range from 200 to 400 nm doesnot lead to any observable loss in efficiency. It has been found thatthis photo-oxidation of the Eu²⁺ ion can be precluded by adding furtherdopants, such as Ce³⁺, Pr³⁺ and Tb³⁺ to the phosphor, said dopants beingthemselves photo-oxidizable.

[0009] In accordance with a preferred embodiment of the invention, thehost lattice is composed of an inorganic material selected from thegroup formed by oxides, sulphides, halogenides, aluminates, gallates,thiogallates, phosphates, borates and silicates.

[0010] It is particularly preferred that the phosphor is selected fromthe group formed by BaMgAl₁₀O₁₇:Eu²⁺,D; BaMgAl₁₀O₁₇:Eu²⁺,Mn,D;(Sr,Ba)MgAl₁₀O₁₇:Eu²⁺,D; BaMgAl₁₆O₂₇:Eu²⁺,D; BaMgAl₁₄O₂₃:Eu²⁺,D;CaAl₁₂O₁₉:Eu²⁺D; SrAl₁₂O₁₉:Eu²⁺,D; SrAl₂O₄:Eu²⁺,D;(Ca,Sr,Ba)₅(PO₄)₃Cl:Eu²⁺,D; Sr₅(PO₄)₃Cl:Eu²⁺,D; Ba₃MgSi₂O₈:Eu²⁺,D;BaF(Cl,Br):Eu²⁺,D and SrGa₂S₄:Eu²⁺,D.

[0011] Within the scope of the invention, it is preferred that thephosphor layer comprises BaMgAl₁₀O₁₇:Eu²⁺,D; Zn₂SiO₄:Mn and(Y,Gd)BO₃:Eu³⁺.

[0012] It may alternatively preferred that the phosphor layer comprisesBaMgAl₁₀O₁₇:Eu²⁺,D; (Y,Gd)BO₃:Tb³⁺ and (Y,Gd)BO₃:Eu³⁺.

[0013] The invention further relates to a phosphor comprising a hostlattice, Eu²⁺ as the activator and a doping D selected from the groupformed by Ce³⁺, Pr³⁺ and Tb³⁺. Such a phosphor can also beadvantageously used for the phosphor layer of colored plasma displayscreens.

[0014] These and other aspects of the invention will be apparent fromand elucidated with reference to two embodiments described hereinafter.

[0015] A gas discharge lamp for dielectrically impeded discharges inaccordance with the invention comprises a discharge vessel containing agas filling, which discharge vessel comprises at least a wall of adielectric material and at least a wall comprising a surface which is atleast partly transparent to visible radiation and coated with a phosphorlayer. Said phosphor layer comprises a phosphor preparation including aphosphor of an inorganic crystalline host lattice, the luminosity ofsaid phosphor being obtained by activating it using an Eu²⁺ dopant. Inaddition, the gas discharge lamp is provided with an electrode structurefor a dielectrically impeded discharge and with means for igniting andmaintaining the dielectrically impeded discharge.

[0016] A typical construction of the gas discharge lamp comprises acylindrical, xenon-filled lamp bulb of glass whose outer wall isprovided with a pair of stripe-shaped electrodes which are arranged soas to be electrically insulated from each other. The stripe-shapedelectrodes extend throughout the length of the lamp bulb so as to faceeach other with their long sides, while leaving two gaps clear. Theelectrodes are connected to the poles of a high-voltage source, which isoperated at an alternating voltage of the order of 20 kHz to 500 kHz insuch a manner that an electric discharge develops only in the region ofthe inside surface of the lamp bulb.

[0017] If an AC voltage is applied to the electrodes, a corona dischargecan be ignited in the xenon-containing filling gas. This leads to theformation of excimers in the xenon, i.e. molecules which consist of anexcited xenon atom and a xenon atom in the ground state.

Xe+Xe*=Xe₂*

[0018] The excitation energy is delivered as UV radiation having awavelength λ=160 to 190 nm. This conversion of electron energy to UVradiation takes place very efficiently. The generated UV photons areabsorbed by the phosphors of the phosphor layer and the excitationenergy is partly delivered again in the region of the spectrum having alonger wavelength.

[0019] In principle, a plurality of different structural shapes arepossible for the discharge vessel, such as plates, simple tubes, coaxialtubes, straight, U-shaped, circularly bent or coiled cylindrical, ordifferently shaped, discharge tubes.

[0020] As the material for the discharge vessel use is made of quartz orglass types.

[0021] The electrodes consist of, for example, a metal, such as aluminumor silver, a metal alloy or a transparent, conductive, inorganiccompound, for example ITO. They can be embodied so as to be a coating,an adhesive foil, wire or wire netting.

[0022] To concentrate the light intensity in a specific direction, apart of the discharge vessel may be provided with a coating serving as areflector for VUV and UV-C light.

[0023] The discharge vessel is filled with a gas mixture comprising aninert gas such as xenon, krypton, neon or helium. Gas fillingscomprising predominantly oxygen-free xenon are preferred.

[0024] The inner wall of the gas discharge vessel is partly, orentirely, coated with a phosphor layer containing one or more phosphorsor phosphor preparations. In addition, the phosphor layer may compriseorganic or inorganic binders or a binder composition.

[0025] For the phosphor use is made of a phosphor comprising the Eu²⁺ion as the activator in a host lattice. The host lattice may be composedof an inorganic material, such as oxides, sulphides, halogenides,aluminates, gallates, thiogallates, phosphates, borates or silicates,said inorganic material being doped with a few percent of the activator.

[0026] The phosphors used in accordance with the invention are first andforemost: MeMgAl₁₀O₁₇:Eu²⁺,D where Me═Ba, Sr having theβ-alumina-crystal structure, for example BaAl₁₀O₁₇:Eu²⁺,D as well as thephosphors having a similar crystal structure, such as BaMgAl₁₆O₂₇:Eu²⁺,Dand BaMgAl₁₄O₂₃:Eu²⁺,D. In addition, use can be made of aluminatephosphors of magnetoplumbite structure MeAl₁₂O₁₉:Eu²⁺,D where Me isselected from the group formed by Ca, Sr, Ba, for exampleBaAl₁₂O₁₉:Eu²⁺,D. These aluminate phosphors are particularly efficientwhen they are excited by vacuum-UV radiation.

[0027] Particularly preferred phosphors are also the green-luminescentstrontium aluminate SrAl₂O₄:Eu²⁺,D; strontium chlorophosphateSr₅(PO₄)₅Cl:Eu²⁺,D; barium magnesium silicate Ba₃MgSi₂O₈:Eu²⁺,D; bariumhalogenide BaF(Cl,Br):Eu²⁺,D and strontium thiogallate SrGa₂S₄:Eu²⁺,D.

[0028] The phosphors activated by Eu²⁺ and doped with Co activatorspreferably comprise 0.1 to 30 mol. % Eu²⁺ and 0.01 to 5 mol. % of thetrivalent Co activators selected from cerium, praseodymium and terbium.

[0029] Phosphors comprising Eu²⁺ as the activator can be readily dopedwith the trivalent Co activators cerium, praseodymium and terbium if,during the manufacture of the phosphors, an oxide selected from thegroup formed by CeO₂, Pr₆O₁₁ and Tb₄O₇ is added to the startingcompounds.

[0030] The absorption coefficient of said phosphors is particularlyhigh, and the quantum yield high, for the wavelengths in the xenonradiation range. The host lattice does not take part in the luminescenceprocess, instead it influences the exact position of the energy levelsof the activator ion and hence the wavelengths of absorption andemission. As an activator in the different host lattices, the Eu²⁺ iongenerally exhibits wide absorption bands in the ultraviolet region whichpartly extend into the blue region. The emission bands extend from thelong ultraviolet up to the yellow-orange region, yet they extendsubstantially in the blue region. Deionization of these phosphors takesplace at a temperature that is approximately slightly above 100° C.

[0031] The grain size of the phosphor particles is not critical.Customarily, use is made of phosphors in the form of fine-grain powderhaving a grain size distribution between 1 and 20 μm.

[0032] For the manufacture of a phosphor layer on a wall of thedischarge vessel use can be made of dry coating methods, such aselectrostatic deposition or electrostatically assisted powdering, aswell as of a wet coating method, such as dip-coating or spraying.

[0033] In the case of the wet coating method, the phosphor preparationmust be dispersed in water, an organic solvent, if necessary inconjunction with a dispersing agent, a surface active agent and ananti-foaming agent, or a binder preparation. Organic or inorganicbinders capable of withstanding an operating temperature of 250° C.without being subject to decomposition, embrittlement or discolorationcan suitably be used as the binder preparations for a gas discharge lampin accordance with the invention.

[0034] The phosphor preparation can be applied, for example, to a wallof the discharge vessel by means of a flow-coating process. The coatingsuspensions used for the flow-coating process comprise water or anorganic compound, such as butylacetate, as the solvent. The suspensionis stabilized by adding auxiliary agents, such as stabilizers,liquefiers, cellulose derivatives, and influenced in its Theologicalproperties. The phosphor suspension is applied in the form of a thinlayer to the walls of the vessel, after which it is dried and fired at600° C.

[0035] It may alternatively be preferred to electrostatically depositthe phosphor preparation for the phosphor layer onto the inside of thedischarge vessel.

[0036] For a gas discharge lamp for dielectrically impeded discharges,which lamp should emit white light, preferably a green-emitting phosphorof the group formed by BaMgAl₁₀O₁₇:Eu²⁺,D; BaMgAl₁₀O₁₇:Eu²⁺,Mn, D and(Ca,Sr,Ba)₅(PO₄)₃Cl: Eu²⁺,D is combined with a red-emitting phosphor ofthe group formed by (Y,Gd)BO₃:Eu³⁺ and Y(V,P)O₄:Eu³⁺ and with agreen-emitting phosphor of the group formed by (Y,Gd)BO₃:Tb³⁺ andZn₂SiO₄:Mn.

[0037] The layer thickness of the phosphor layer customarily ranges from5 to 100 μm.

[0038] The vessel is subsequently evacuated so as to remove all gaseousimpurities, in particular oxygen. Next, the vessel is filled with xenonand sealed.

EXAMPLE 1

[0039] A cylindrical discharge vessel of glass having a length of 590nm, a diameter of 10 nm and a wall thickness of 0.8 nm is filled withxenon at a pressure of 200 hPa. The discharge vessel comprises an innerelectrode, which extends parallel to the axis, and which is in the formof a rod of a noble metal having a diameter of 2.2 mm. The outsidesurface of the discharge vessel is provided with the outer electrode inthe form of two conductive silver strips having a width of 2 mm each,which strips are arranged so as to be parallel to the axis andconductively connected to the power supply. The lamp is operated bymeans of a pulsed AC voltage. The inside wall of the discharge vessel iscoated with a phosphor layer. Said phosphor layer comprises athree-banded phosphor mixture of the following components:BaMgAl₁₀O₁₇:Eu²⁺,Pr³⁺ as the blue component, Y₂O₃:Eu as the redcomponent and Y₂SiO₅:Tb as the green component.

[0040] To manufacture said BaMgAl₁₀O₁₇:Eu²⁺,Pr³⁺, 69.3 g (0.350 mol)BaCO₃, 7.0 g (0.03 mol) Eu₂O₃, 4.0 g (4.0 mol) Pr₆O₁₁, 11.2 g (0.278mol) MgO, 204 g (2.0 mol Al₂O₃) and 7.4 g (0.119 mol ) MgF₂ arethoroughly mixed and ground in an agate mortar. Said mixture is sinteredin a reducing atmosphere of nitrogen with 1% hydrogen at 1300° C. for 2hours. The sintered powder is ground again and sieved so as to obtain aparticle size d<36 μm.

[0041] In this manner a light output of initially 37 lm/W was achieved.After 1000 working hours, the light output was approximately 34 lm/W.The quantum yield for VUV light is approximately 70%.

EXAMPLE 2

[0042] A cylindrical discharge vessel of glass having a length of 590nm, a diameter of 10 nm and a wall thickness of 0.8 nm is filled withxenon at a pressure of 200 hPa. The discharge vessel comprises an innerelectrode, which extends parallel to the axis, and which is in the formof a rod of a noble metal having a diameter of 2.2 mm. The outsidesurface of the discharge vessel is provided with the outer electrode inthe form of two conductive silver strips having a width of 2 mm each,which strips are arranged so as to be parallel to the axis andconductively connected to the power supply. The lamp is operated bymeans of a pulsed AC voltage.

[0043] The inside wall of the discharge vessel is coated with a phosphorlayer. Said phosphor layer comprises a three-banded phosphor mixture ofthe following components: BaMgAl₁₀O₁₇:Eu²⁺,Ce³⁺ as the blue component,(Y, Gd)BO₃:Eu³⁺ as the red component and Zn₂SiO₄:Mn as the greencomponent.

[0044] To manufacture said BaMgAl₁₀O₁₇:Eu²⁺,Ce³⁺, 69.3 g (0.350 mol)BaCO₃, 7.0 g (0.03 mol) Eu₂O₃, 4.1 g (24.0 mmol) CeO₂, 11.2 g (0.278mol) MgO, 204 g (2.0 mol Al₂O₃) und 7.4 g (0.119 mol ) MgF₂ arethoroughly mixed and ground in an agate mortar. Said mixture is sinteredin a reducing atmosphere of nitrogen with 1% hydrogen at 1300° C. for 2hours. The sintered powder is ground again and sieved so as to obtain aparticle size d<36 μm.

[0045] In this manner a light output of initially 37 lm/W was achieved.After 1000 working hours, the light output was approximately 34 lm/W.The quantum yield for VUV light is approximately 70%.

1. A gas discharge lamp for dielectrically impeded discharges, which gasdischarge lamp is provided with a discharge vessel filled with a gasfilling, which discharge vessel comprises at least a wall of adielectric material and at least a wall having a surface which is atleast partly transparent to visible radiation and coated with a phosphorlayer, which phosphor layer comprises a phosphor having a host lattice,Eu²⁺ as the activator and a doping D selected from the group formed byCe³⁺, Pr³⁺ and Tb³⁺, and provided with an electrode structure for adielectrically impeded discharge and means for igniting and maintainingthe discharge.
 2. A gas discharge lamp as claimed in claim 1,characterized in that the host lattice is composed of an inorganicmaterial selected from the group formed by oxides, sulphides,halogenides, aluminates, gallates, thiogallates, phosphates, borates andsilicates.
 3. A gas discharge lamp as claimed in claim 1, characterizedin that the phosphor is selected from the group formed byBaMgAl₁₀O₁₇:Eu²⁺,D; BaMgAl₁₀O₁₇:Eu²⁺,Mn,D; (Sr,Ba)MgAl₁₀O₁₇:Eu²⁺,D;BaMgAl₁₆O₂₇:Eu²⁺,D; BaMgAl₁₄O₂₃:Eu²⁺,D; CaAl₁₂O₁₉:Eu²⁺D;SrAl₁₂O₁₉:Eu²⁺,D; SrAl₂O₄:Eu²⁺,D; (Ca,Sr,Ba)₅(PO₄)₃Cl:Eu²⁺,D;Sr₅(PO₄)₃Cl:Eu²⁺,D; Ba₃MgSi₂O₈:Eu²⁺,D; BaF(Cl,Br):Eu²⁺,D andSrGa₂S₄:Eu²⁺,D.
 4. A gas discharge lamp as claimed in claim 1,characterized in that the phosphor layer comprises BaMgAl₁₀O₁₇:Eu²⁺,D;Zn₂SiO₄:Mn²⁺ and (Y,Gd)BO₃:Eu³⁺.
 5. A gas discharge lamp as claimed inclaim 1, characterized in that the phosphor layer comprisesBaMgAl₁₀O₁₇:Eu²⁺,D; (Y,Gd)BO₃:Tb³⁺ and (Y,Gd)BO₃:Eu³⁺.
 6. A phosphorcomprising a host lattice, Eu²⁺ as the activator and a doping D selectedfrom the group formed by Ce³⁺, Pr³⁺ and Tb³⁺.